The present invention relates to fiber optics transmitters and receivers for use in two-way communication systems.
Information can be transmitted through cables via electrical current, microwaves or light waves. The bandwidth of the information which can be carried by modulating the electrical current is limited. Higher bandwidth data transmission can be provided by microwave propagation. Of course, the use of light waves provides the highest bandwidth, which can be into the gigahertz range.
Light wave communication can be done through space or through light pipes or a waveguide. For example, many of the remote control boxes for television sets use an infared beam to send a control signal to the television set. For voice, video or data communication, an optical fiber is used as a waveguide for the light wave. An optical fiber is made up of different mixtures of silicon dioxide material and typically has a diameter of approximately 100 micrometers. As a result, the optical cable is very light weight. In an optical communication system, light from a light source, such as a light emitting diode (LED) or laser diode, is coupled into the core of the optical fiber. Since the optical fiber is made up of low loss material, most of the light coupled into the optical fiber will emerge again on the other end of the fiber. A high speed photodetector can be used to pickup the information and convert it back into electrical signals. At present, most fiber optics communication systems are designed to use one fiber for transmitting information and one fiber for receiving information. The number of fibers bundled into a fiber cable is determined by the number of transmitters and receivers required at both destinations.
Because of the ever increasing volume of information to be communicated over large distances, there is an increasing need for using optical fiber as the channel for communication. In many offices today, microcomputers, minicomputers and mainframe computers are connected into a local area network. While coaxial cables are employed extensively for connections within a building, separated buildings are now linked together by coaxial cables or optical fibers. Although optical fibers can carry more information, the cost of optical fibers limits the wide spread use of fiber optics in networking applications.
FIG. 1(A) shows an optical fiber with a transmitter at one end of the fiber and a receiver at the other end. A light source 1, which is either a light emitting diode or a laser diode, emits a cone of light. A lens 2 is used to collect the light and image it to one end of an optical fiber 3. The amount of light coupled into the optical fiber is determined by the numerical aperture of lens 2 and the amount of divergence of the cone of the light source. Except for a small amount of light loss through scattering by the particulate matter inside the waveguide, most of the light is transmitted through the fiber and focused by a lens 4 to a detector 5. The light loss through the fiber is a strong function of the wavelength of the light. For wavelengths in the near infared region from 780 nm to 850 nm, the rate of light loss is about 2 db/Kilometer. The loss is even lower at 1300 nm wavelength. Hence, information can be communicated over a few tens of kilometers without the need of a repeater. FIG. 1(B) shows a pair of optical fibers for a two-way communication system.
FIG. 2 shows another two-way communication system using a single optical fiber. A laser diode 6 emits a diverging light beam which is collimated or made parallel by a collimating lens 7. A second lens 8 is used to focus the collimated beam into a small light spot in order to couple the laser light into an optical fiber 12. A beam-splitter 9 is interposed between the lenses 7 and 8 to reflect a portion of the light coming out from the end of the fiber. This portion of the returned light is focused by lens 10 into a detector 11. An identical system is also implemented on the other end of the optical fiber. This two-way communication system eliminates the need for using two optical fibers. However, the alignment of the optical components to couple light into and out of the optical fiber becomes not only more critical, but also more sensitive to mechanical vibration and other environmental changes.
The amount of information carried by a single optical fiber can be increased by time multiplexing the signals or frequency multiplexing the signals. When the signals are frequency multiplexed, two or more different wavelength light beams are used. This requires an optical apparatus at the receiving end to separate the two beams, or demultiplex them. Some systems use diffraction gratings created by holographic methods to diffract a beam of one wavelength while allowing the beam of the other wavelength to continue on its original path. Such demultiplexers or multiplexers are shown, for instance, in U.S. Pat. Nos. 3,666,345; 4,198,117; 4,359,259; 4,362,359; 4,387,955; and 4,626,069. Hologram lenses are also used in coupler devices which serve a repeater function in a fiber optic cable. Examples are shown in U.S. Pat. Nos. 3,975,082; 4,057,319; and 4,465,332.
The objective of this invention is to disclose an optical transmitter/receiver device which can provide two-way communication through a single fiber without the disadvantages of the prior art systems.